Well, I've done it...and I won't do it again.
Here's the skinny...
When you "buff" it, you accomplish nothing.
To remove even the slightest nick, you must use an
abrasive and actually "grind down" (polish) the glass around
the nick to make it disappear. Doing so changes the optics.
In other words, the glass will look distorted under
certain lighting conditions. Not acceptable.
Here is the test. If you can "catch" the nick or the scratch
with a fingernail, replace it. Period. If you cannot "catch" the
flaw, buff it out.
You can be a LITTLE less picky on the side glass, but be
careful messing with the windshield. I recently "polished"
out some scratches on my one side window, and was unhappy enough
with the results I replaced the glass.
As always, ymmv and I could be wrong.
Paul